You need to check with the specific nightclub you are planning to attend to see what their policies are.
A lot of places in USA with alcohol are 21 and over, because the drinking age nowadays is usually 21.
Some clubs will let people 18 and over in, but depending on the club and the location, those under 21 may (or may not) be barred from certain parts of the club.
Some clubs will have certain nights when it’s 18 and over and other nights when it’s 21 and over.
I would hope if your friends are inviting you to join them, they will pick a place where you are allowed to go in with them and spend time with them, not be shunted off into some separate area.